Added support for embedded headings and changes to media postprocessor

[tnewing]

Added a new preprocessor, "EmbedPreprocessor", which replaces heading internal embeds with the corresponding markdown. The effect is that a flashcard such as the following;

id: 1691710051726
---
What is Bayes theorem?
===
![Theorem](#Theorem)

will now generate a flashcard containing the embedded content as the back - essentially frees you from having to write notes into the code block directly, keeping the body of your notes as the ground truth. Currently only works for markdown links. Also the embedded content doesn't render in preview mode yet - might fix these later.

Also made a minor change to the media processor, which was having trouble identifying and locating internally embedded files.

[zinc0x1E]

Hi @tnewing , I love your ideas and it seems like this repo is not activly maintained. So would you mind me to merge your changes to my own fork? I really want to taste these features right now